Secondary battery system, charging method, and vehicle

ABSTRACT

According to one embodiment, a secondary battery system includes a secondary battery, a first measurement part, a designation part, and a controller. The first measurement part measures a volume change of the secondary battery. The designation part designates a threshold value. The controller controls a current flowing through the secondary battery, based on the volume change of the secondary battery measured by the first measurement part and the threshold value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2017-086462, filed on Apr. 25, 2017; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a secondary batterysystem, a charging method, and a vehicle.

BACKGROUND

With spread of an information-related device and a communication device,a secondary battery has been widely spread as a power source of adevice. The secondary battery is also used in a field of an electricvehicle (EV) and natural energy. In particular, a lithium ion secondarybattery has high energy density and can be miniaturized, and thereforeis widely used.

In the lithium ion secondary battery, an active material used for apositive electrode and a negative electrode occludes and releaseslithium ions to store and release electric energy. During charging,lithium ions released from a positive electrode are occluded by anegative electrode. During discharging, lithium ions released from anegative electrode are occluded by a positive electrode.

In a method for charging a secondary battery, in a case where thebattery is charged with a voltage higher than a set voltage, the batteryis deteriorated significantly. Therefore, constant current-constantvoltage (CC-CV) charging for controlling a current so as to keep a setvoltage after charging is performed to a predetermined voltage by aconstant current is performed.

In order to shorten charging time, it is conceivable to set a currentvalue of a constant current (constant electric power) to a high valuewhile charging is performed at a constant current (constant electricpower). However, a high current value deteriorates storage batteryperformance of a secondary battery, such as a battery capacity orinternal resistance. Furthermore, lifetime of the secondary battery isshortened disadvantageously due to deterioration of the storage batteryperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of a secondary batterysystem according to the first embodiment.

FIGS. 2A and 2B are a cross-sectional view showing an example of a unitbattery mounted on a storage battery 2 and an enlarged view thereof.

FIG. 3 is a flowchart showing an example of operation of the secondarybattery system according to the first embodiment.

FIG. 4 is a block diagram showing an example of a secondary batterysystem according to the second embodiment.

FIG. 5 is a graph showing characteristics in a case where electromotiveforces of active materials are different from each other.

FIG. 6 is a graph obtained by deriving characteristics of anelectromotive voltage of a composite positive electrode obtained bymixing an active material A and an active material B with respect to acharge amount.

FIG. 7 is a flowchart showing an example of operation of the secondarybattery system according to the second embodiment.

FIG. 8 is a block diagram showing an example of a secondary batterysystem according to the third embodiment.

FIG. 9 is a graph showing a relationship between a charge amount and thethickness of a unit battery in a case where graphite is used for anegative electrode of a unit battery.

FIG. 10 is a flowchart showing an example of operation of the secondarybattery system according to the third embodiment.

FIG. 11 is a graph showing a result of a cycle test performed bychanging a ratio of I₁/I₃ in a case where graphite is used for anegative electrode of a storage battery.

FIG. 12 is a graph showing a relationship between the number of cyclesand a capacity retention ratio in a case of 0.7C-CCCV charging and0.8C-0.9C-0.6CCV charging.

FIG. 13 is a diagram showing an example of a vehicle according to thefourth embodiment.

DETAILED DESCRIPTION

According to one embodiment, a secondary battery system includes asecondary battery, a first measurement part, a designation part, and acontroller. The first measurement part measures a volume change of thesecondary battery. The designation part designates a threshold value.The controller controls a current flowing through the secondary battery,based on the volume change of the secondary battery measured by thefirst measurement part and the threshold value.

Hereinafter, a secondary battery system, a charging method, and avehicle according to embodiments will be described with reference to thedrawings. Those with the same reference numeral indicate similar items.Note that the drawings are schematic or conceptual, and a relationshipbetween the thickness and the width of each portion, a ratio coefficientof the size between the portions, and the like are not necessarily thesame as actual ones. Even in a case of indicating the same portion, thedimensions and ratio coefficients of the portion may be different fromeach other depending on the drawing.

First Embodiment

The first embodiment will be described with reference to FIG. 1. FIG. 1is a block diagram showing an example of a secondary battery systemaccording to the first embodiment.

As shown in FIG. 1, a secondary battery system 1 includes a storagebattery 2, a volume measurement part 3 (also referred to as a firstmeasurement part), a charge controller 4, and a designation part 5.

In the secondary battery system 1, the volume measurement part 3measures a volume change of the storage battery 2, and the chargecontroller 4 controls the amount of a current flowing through thestorage battery 2, based on the volume change of the storage battery 2and a threshold value designated by the designation part 5.

First, the storage battery 2 will be described. The storage battery 2 isa battery charged by the charge controller 4. The storage battery 2includes one or more battery packs. Each battery pack includes one ormore battery modules (also referred to as assembled batteries), and eachbattery module includes one or more unit batteries (also referred to ascells). The number of battery modules included in each battery pack maybe different among the battery packs. In addition, the number of unitbatteries included in each battery module may be different among thebattery modules. As a unit battery, a chargeable/dischargeable secondarybattery is used. For example, the unit battery is preferably a lithiumion secondary battery.

FIGS. 2A and 2B are a cross-sectional view showing an example of a unitbattery mounted on the storage battery 2 and an enlarged view thereof.As shown in FIGS. 2A and 2B, a unit battery includes an exterior member20 and a flat wound electrode group 21 housed in the exterior member 20.The wound electrode group 21 has a structure in which a positiveelectrode 22 and a negative electrode 23 are spirally wound with aseparator 24 interposed therebetween. A nonaqueous electrolyte (notshown) is held by the wound electrode group 21. As shown in FIGS. 2A and2B, the negative electrode 23 is located on an outermost periphery ofthe wound electrode group 21. The positive electrode 22 and the negativeelectrode 23 are alternately laminated via the separator 24 in such amanner that the separator 24, the positive electrode 22, the separator24, the negative electrode 23, the separator 24, the positive electrode22, and the separator 24 are located on an inner peripheral side of thenegative electrode 23. The negative electrode 23 includes a negativeelectrode current collector 23 a and a negative electrode activematerial-containing layer 23 b supported by the negative electrodecurrent collector 23 a. In a portion located on an outermost peripheryof the negative electrode 23, the negative electrode activematerial-containing layer 23 b is formed only on one side of thenegative electrode current collector 23 a. The positive electrode 22includes a positive electrode current collector 22 a and a positiveelectrode active material-containing layer 22 b supported by thepositive electrode current collector 22 a. As shown in FIGS. 2A and 2B,a belt-like positive electrode terminal 25 is electrically connected tothe positive electrode current collector 22 a near an outer peripheralend of the wound electrode group 21. Meanwhile, a belt-like negativeelectrode terminal 26 is electrically connected to the negativeelectrode current collector 23 a near the outer peripheral end of thewound electrode group 21. Tips of the positive electrode terminal 25 andthe negative electrode terminal 26 are led out to an outside from thesame side of the exterior member 20.

For the exterior member 20 used for a unit battery, a metallic containeror a container made of a laminate film is used. As the metalliccontainer, a square or cylindrical metallic can made of aluminum, analuminum alloy, iron, stainless steel, or the like is used.

The positive electrode 22 used for a unit battery includes the positiveelectrode current collector 22 a and the positive electrode activematerial-containing layer 22 b. The positive electrode activematerial-containing layer 22 b is formed on one side or both sides ofthe positive electrode current collector 22 a, and includes an activematerial, a conductive agent, and a binder. As the positive electrodeactive material, for example, an oxide and a composite oxide are used.The oxide includes an oxide represented by either of the followingformulas (i) and (ii).

LiNi_(x)M1_(y)O₂  (i)

LiMn_(u)M2_(v)O₄  (ii)

M1 is at least one element selected from the group consisting of Mn, Co,Al, Ti, Zr, Cr, V, and Nb. In x+y=1, 0<x≤1.0 and 0≤y≤1.0 are satisfied.M2 is at least one element selected from the group consisting of Al, Mg,Ti, Zr, Cr, V, and Nb. In u+v=2, 0<u≤2.0 and 0≤v<2.0 are satisfied. Aconductive agent enhances current collection performance and suppressescontact resistance between an active material and a current collector.Preferable examples of the conductive agent include acetylene black,carbon black, graphite, and a carbon fiber. A binder binds an activematerial, a conductive agent, and a current collector. Preferableexamples of the binder include polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF), and a fluorine-based rubber. Thepositive electrode current collector 22 a is preferably an aluminum foilor an aluminum alloy foil containing one or more elements selected fromthe group consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si.

The negative electrode 23 used for a unit battery includes the negativeelectrode current collector 23 a, and the negative electrode activematerial-containing layer 23 b formed on one side or both sides of thenegative electrode current collector 23 a and containing an activematerial, a conductive agent, and a binder. Examples of the negativeelectrode active material include a lithium titanium oxide representedby a general formula Li_(4/3+x)Ti_(5/3)O₄ (0≤x), a monoclinic materialrepresented by a general formula Li_(x)TiO₂ (0≤x) (bronze structure B),a titanium oxide having an anatase structure (TiO₂ as a structure beforecharge), and a niobium titanium oxide represented by a general formulaLi_(x)Nb_(a)TiO₇ (0≤X, 1≤a≤4). In addition, a lithium titanium oxide(lithium titanium-containing composite oxide) having a ramsdellitestructure, such as Li_(2+x)Ti₃O₇, Li_(1+x)Ti₂O₄, Li_(1.1+x)Ti_(1.8)O₄,Li_(1.07)+Ti_(1.86)O₄, or Li_(x)TiO₂ (0≤x) can also be used. Inaddition, a lithium titanium oxide or a titanium oxide containing atleast one element selected from the group consisting of Nb, Mo, W, P, V,Sn, Cu, Ni and Fe may be used. A lithium titanium oxide represented byLi_(x)TiO₂ or Li_(4/3+x)Ti_(5/3)O₄ (0≤x≤2) is preferable. As a negativeelectrode, a new material containing orthorhombic Na-containing niobiumtitanium composite oxide particles Li₂Na_(2−x)Ti_(6−x)Nb_(x)O₁₄ (LNT)having a high capacity may be used. Each of these negative electrodeactive materials may contain graphite. As the negative electrode currentcollector 23 a, an aluminum foil or an aluminum alloy foil ispreferable. Preferable examples of the conductive agent includeacetylene black, carbon black, coke, a carbon fiber, graphite, metalcompound powder, and metal powder. Preferable examples of the binderinclude polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF),a fluorine-based rubber, a styrene-butadiene rubber, and a core-shellbinder.

As the separator 24 used for a unit battery, an olefin-based porous filmsuch as polyethylene (PE) or polypropylene (PP) having a porosity of 50%or more, or a cellulose fiber is preferable. Examples thereof include anonwoven fabric, a film, and paper having a fiber diameter of 10 μm orless.

Examples of the nonaqueous electrolyte include a liquid nonaqueouselectrolyte prepared by dissolving an electrolyte in an organic solvent,a gelatinous nonaqueous electrolyte which is a composite of a liquidelectrolyte and a polymer material, and a solid nonaqueous electrolytewhich is a composite of a lithium salt electrolyte and a polymermaterial. In addition, an ambient temperature molten salt (ionic melt)containing lithium ions may be used as the nonaqueous electrolyte.Examples of the polymer material include polyvinylidene fluoride (PVdF),polyacrylonitrile (PAN), and polyethylene oxide (PEO).

A plurality of unit batteries included in a battery pack can beelectrically connected in series, in parallel, or in combination ofseries and parallel. The plurality of unit batteries can be electricallyconnected to constitute an assembled battery. The battery pack mayinclude a plurality of assembled batteries. Note that the term storagebattery includes a battery pack, a battery module (assembled battery), aunit battery, and a secondary battery.

Next, the volume measurement part 3 will be described. The volumemeasurement part 3 measures a volume change in each unit battery of thestorage battery 2. The volume measurement part 3 is disposed on asurface of the exterior member 20 of a unit battery. In the storagebattery 2 according to the present embodiment, a current flows duringcharging, and therefore each unit battery expands or contracts accordingto a charging state. This is because the volume of an active materialchanges by occluding and releasing ions when the storage battery 2performs charge and discharge. The volume change of a unit battery givesa dynamic load to an active material, causes physical breakage (forexample, cracking) of the active material, and therefore deterioratesperformance of the unit battery. In addition, the volume change of aunit battery is caused by occlusion and release of ions, and therefore avolume change rate increases when a current is large. Meanwhile, in acase where the current is small, the volume change rate is small. Thevolume measurement part 3 measures a volume change at the time ofexpansion and contraction of each unit battery. The volume change to bemeasured may be a volume change rate per unit time or a volume changeamount (difference). Furthermore, the volume change also includes achange in the thickness of a unit battery or the like. The number of theunit batteries measured by the volume measurement part 3 is not limitedto a plurality, but may be one. For example, a displacement gauge, astrain gauge, a pressure sensor, or the like for measuring the thicknessof a unit battery is used as the volume measurement part 3. Adisplacement in one direction of a unit battery may be measured andconverted to a volume change. In a case where a unit battery has asquare shape, the volume measurement part 3 preferably performsmeasurement at a portion having the largest surface area. A volumechange measured by the volume measurement part 3 is output to the chargecontroller 4. Volume change data measured by the volume measurement part3 does not need to be output to the charge controller 4 all the time,and may be output at predetermined time intervals.

Next, the charge controller 4 will be described. The charge controller 4controls a current flowing through the storage battery 2, based on ameasurement result of the volume measurement part 3 and a thresholdvalue designated by the designation part 5 described later. The chargecontroller 4 includes a second measurement part 40 and a deriving part41.

The second measurement part 40 measures information such as the current,voltage, temperature, and the like of the storage battery 2, and outputsthe information to the deriving part 41. The second measurement part 40may be a sensor or the like capable of measuring a current, a voltage,and a temperature.

The deriving part 41 controls a current flowing through the storagebattery 2 such that a volume change rate of the storage battery 2 doesnot exceed a threshold value, based on the volume change rate of thestorage battery 2 measured by the volume measurement part 3, theinformation such as a current, a voltage or a temperature measured bythe second measurement part 40, and a threshold value designated by thedesignation part 5 described later. More specifically, while the chargecontroller 4 causes a predetermined current to flow through the storagebattery 2, in a case where the volume change of the storage battery 2 isequal to or more than a threshold value, the charge controller 4controls a current flowing through the storage battery 2 so as to besmaller than the predetermined current. In a case where the volumechange of the storage battery 2 is equal to or less than the thresholdvalue, the charge controller 4 controls the current flowing through thestorage battery 2 so as to be larger than the predetermined current. Thepredetermined current indicates a current flowing when a storage batteryis charged. The predetermined current is arbitrarily set according toempirical rules and characteristics of the storage battery. For example,a constant current during constant current-constant voltage (CC-CV)charging corresponds to the predetermined current. The charge controller4 causes a predetermined current to flow in an initial charging state ofthe storage battery 2. In a case where the volume change of the storagebattery 2 does not exceed a threshold value, the charge controller 4controls a current so as to be larger than the predetermined current. Ina case where the volume change of the storage battery 2 exceeds thethreshold value, the charge controller 4 controls the current so as tobe smaller than the predetermined current. Furthermore, the chargecontroller 4 judges whether a predetermined charge amount has beensatisfied by deriving a charge amount, based on a value of a currentflowing through the storage battery 2. It is only required to judgewhether a predetermined charge amount has been satisfied based on presetconditions. The deriving part 41 includes, for example, a centralprocessing unit (CPU), a memory, and an auxiliary storage, and executesa program or the like. Note that all or a part thereof may be realizedby using a hardware such as an application specific integrated circuit(ASIC), a programmable logic device (PLD), or a field programmable gatearray (FPGA).

The charge controller 4 may arbitrarily set time for causing apredetermined current to flow through the storage battery 2. Even if apredetermined current flows through the storage battery 2 for a longtime, a volume change of the storage battery 2 may be smaller than athreshold value. Therefore, it is only required to control a currentsuch that the current increases after a predetermined current has flowedfor a certain time. A case where the second measurement part 40 isincluded in the charge controller 4 has been described. However, thesecond measurement part 40 may be configured separately from the chargecontroller 4 without being limited to this case.

Next, the designation part 5 will be described. The designation part 5designates a threshold value for a volume change of the storage battery2. In a case where the charge controller 4 performs control based on avolume change rate of the storage battery 2 per unit time, a thresholdvalue of the volume change rate is input to the designation part 5. Thethreshold value input to the designation part 5 is arbitrarily set by anoperator (user) according to past data and characteristics of thestorage battery 2. For example, the threshold value is preferably set toa value equal to or lower than a volume change rate at which the storagebattery 2 is significantly deteriorated.

The designation part 5 may designate a threshold value using an externalterminal such as a personal computer (PC) or a mobile phone, or mayinput the threshold value directly by attaching a monitor, a touchpanel, or the like. In a case of designation by an external terminal,data may be transferred using the Internet, Wi-Fi, Bluetooth (registeredtrademark), or the like.

The threshold value designated by the designation part 5 is output tothe charge controller 4. In a case where the threshold value is clearlydetermined by characteristics of the storage battery 2, there is no needto input a threshold value by the designation part 5, but the chargecontroller 4 only needs to have information of the threshold value. Inthis case, the designation part 5 is not an essential component.Alternatively, the charge controller 4 may include the designation part5. Furthermore, the threshold value input to the designation part 5 isnot limited to a fixed value, but may be a variation value according tocharging time, for example.

Next, an example of operation of the secondary battery system accordingto the first embodiment will be described. FIG. 3 is a flowchart showingan example of operation of the secondary battery system according to thefirst embodiment.

First, information of a volume change rate of the storage battery 2 perunit time is designated to the designation part 5 (step 301). In a casewhere the charge controller 4 previously has information of the volumechange rate of the storage battery 2, this step is omitted.

The charge controller 4 causes a predetermined current to flow throughthe storage battery 2 (step 302).

The volume measurement part 3 measures a volume change of the storagebattery 2 and outputs the volume change to the charge controller 4 (step303).

The charge controller 4 compares a threshold value input to thedesignation part 5 with a volume change rate of the storage battery 2(step 304).

If the volume change rate of the storage battery 2 is equal to or largerthan the threshold value (Yes in step 304), the charge controller 4controls a current flowing through the storage battery 2 so as to besmaller than a predetermined current such that the volume change rate issmaller than the threshold value (step 305).

If the volume change rate of the storage battery 2 is smaller than thethreshold value (No in step 304), the charge controller 4 controls thecurrent flowing through the storage battery 2 so as to be larger thanthe predetermined current such that the volume change rate approachesthe threshold value (step 306).

The charge controller 4 judges whether the storage battery 2 is fullycharged (100% charge) (step 307).

If the charge controller 4 judges that the storage battery 2 is notfully charged (No in step 307), the process returns to step 303.

If the charge controller 4 judges that the storage battery 2 is fullycharged (Yes in step 307), the charge controller 4 stops supply of acurrent to the storage battery 2 (step 308). Thereafter, the process isterminated.

By using the secondary battery system 1 according to the firstembodiment, a current can be controlled according to a volume change ofthe storage battery 2 during charging. Therefore charging with reduceddeterioration of the storage battery 2 can be performed.

Furthermore, the volume measurement part 3 can measure the volume changeof the storage battery 2 in a timely manner. Therefore, efficientcurrent control can be performed, and charging time can be shortened.

Furthermore, by using a plurality of identical unit batteries for thestorage battery 2, it is possible to estimate a volume change of anotherunit battery by measuring a volume change of one unit battery.Therefore, a secondary battery system can be realized with a simpleconfiguration.

Second Embodiment

The second embodiment will be described with reference to FIG. 4. FIG. 4is a block diagram showing an example of a secondary battery systemaccording to the second embodiment.

As shown in FIG. 4, a secondary battery system 1 includes a storagebattery 2, a charge controller 4, a designation part 5, an estimationpart 6, and a storage 7. The secondary battery system according to thesecond embodiment includes the estimation part 6 and the storage 7instead of the volume measurement part 3. Other configurations aresimilar to those of the secondary battery system according to the firstembodiment.

In the secondary battery system 1 according to the second embodiment,the estimation part 6 estimates a volume change of the storage battery2, based on data measured by a second measurement part 40 and datapreviously stored in the storage 7. The charge controller 4 controls acurrent flowing through the storage battery 2, based on the volumechange of the storage battery 2 estimated by the estimation part 6 and athreshold value designated by the designation part 5.

First, the estimation part 6 will be described in detail. The estimationpart 6 includes a CPU 61, a RAM (RWM) 62, a communication IF 63, a ROM64, and a storage 65. In addition, the estimation part 6 may include aninterface (IF) on which an external storage device such as a USB memoryis mounted. The estimation part 6 is a computer for executing andcomputing a program.

The estimation part 6 collects data such as a current or a voltage ofthe storage battery 2 measured by the second measurement part 40 via thecommunication IF 63, and performs various deriving processes using thecollected data.

The CPU 61 is an arithmetic processing unit (microprocessor) for readingeach program previously written in the ROM 64 into the RAM 62 andperforming a deriving process. The CPU 61 can be configured by aplurality of CPU groups (microcomputers and microcontrollers) accordingto a function. In addition, the CPU 61 may include a built-in memoryhaving a RAM function.

The RAM (RWM) 62 is a memory area used when the CPU 61 executes aprogram, and is a memory used as a working area. The RAM (RWM) 62preferably temporarily stores data necessary for a process.

The communication IF 63 is a communication device and a communicationmeans for exchanging data with the charge controller 4. For example, thecommunication IF 63 is a router. In the second embodiment, connectionbetween the communication IF 63 and the storage battery 2 is describedas wired communication, but can be replaced with various wirelesscommunication networks. Furthermore, connection between thecommunication IF 63 and the charge controller 4 may be performed via anetwork capable of one-way or two-way communication.

The ROM 64 is a program memory for storing an estimation program 641. Anon-primary storage medium on which data cannot be written is preferableused. However, a storage medium from which data can be read and on whichdata can be written at any time, such as a semiconductor memory, may beused. Furthermore, the ROM 64 may store an information registrationprogram or the like for storing acquired data in the storage 65 atpredetermined time intervals.

The estimation program 641 is a means for causing the CPU 61 to realizea function of deriving capacity values and internal resistance values ofa positive electrode and a negative electrode for each unit battery oreach assembled battery constituting the storage battery 2. For example,seven values represented by numerical formula (1), (a) a capacity of anactive material A constituting a positive electrode, (b) a capacity ofan active material B constituting the positive electrode, (c) a capacityof a negative electrode, (d) a charge amount of the active material Aconstituting the positive electrode, (e) a charge amount of the activematerial B constituting the positive electrode, (f) a charge amount ofthe negative electrode, and (g) an internal resistance value, arederived (analyzed).

Q _(cA)

Q _(cB)

Q _(a)

q ₀ ^(cA)

q ₀ ^(cB)

q ₀ ^(a) R  (1)

By using these values, change characteristics of a charging voltage withrespect to time, and potential characteristics of the positive electrodewith respect to the charge amount or the negative electrode with respectto the charge amount, are derived. Specific operation will be describedlater.

The estimation program 641 is configured by a program groupcorresponding to each of the following numerical formulas. Note thatorder of the programs can be changed variously.

A charging voltage V_(C) is determined from the following numericalformula (2) using an electromotive voltage V_(e) of a battery, and avoltage V_(R) due to internal resistance.

V _(C) =V _(e) +V _(R)  (2)

The electromotive voltage V_(e) of a battery is determined from thefollowing numerical formula (3) using a potential E_(c) of a positiveelectrode and a potential E_(a) of a negative electrode.

V _(e) =E _(c) −E _(a)  (3)

The potentials of the positive electrode and the negative electrode aredetermined from numerical formulas (4) and (5) using a charge amount(q), a capacity Q_(ic) of the positive electrode in an initial state,and a capacity Q_(ia) of the negative electrode in the initial state.

E _(c) =f _(c)(q/Q _(ic))  (4)

E _(a) =f _(a)(q/Q _(ia))  (5)

Here, a case where a positive electrode or a negative electrode isconstituted by a plurality of active materials will be described.

FIG. 5 is a graph showing characteristics in a case where electromotiveforces of active materials are different from each other.

Characteristics of an electromotive voltage of a composite positiveelectrode obtained by mixing the active material A (for example, lithiummanganate) and the active material B (for example, lithium cobaltite)with respect to a change amount are derived.

FIG. 6 is a graph obtained by deriving characteristics of anelectromotive voltage of a composite positive electrode obtained bymixing the active material A and the active material B with respect to acharge amount.

A potential E_(cA) of a positive electrode of the active material A anda potential E_(cB) of a positive electrode of the active material B aredetermined from numerical formulas (6) to (9) using a capacity Q_(icA)of the active material A in an initial state, a capacity Q_(icB) of theactive material B in an initial state, a charge amount q_(A) of theactive material A, and a charge amount q_(B) of the active material B.

E _(cA) =f _(cA)(q _(A) /Q _(icA))  (6)

E _(cB) =f _(cB)(q _(B) /Q _(icB))  (7)

f _(cA)(q _(A) /Q _(cA))=f _(cB)(q _(B) /Q _(cB))  (8)

q=q _(A) +q _(B)  (9)

Therefore, the potential E_(c) of a mixed positive electrode isdetermined from numerical formula (10) using the capacity q_(A) at thetime of starting charging the positive electrode of the active materialA and the charge amount Q_(cA) of the positive electrode of the activematerial A, or the capacity q_(B) at the time of starting charging thepositive electrode of the active material B and the charge amount Q_(cB)of the positive electrode of the active material B.

E _(c) =f _(c)(q/Q _(ic))=f _(cA)(q _(A) /Q _(cA))=f _(cB)(q _(B) /Q_(cB))  (10)

Note that the potential E_(cA) of the positive electrode of the activematerial A and the potential E_(cB) of the positive electrode of theactive material B are potentials of surfaces of the respective activematerials. Therefore, distribution of lithium ions in an active materialis changed according to diffusion resistance of lithium ions in theactive material. As a result, it seems that a relationship between acharge amount and an electromotive voltage changes according to acharging current. However, in the second embodiment, in an activematerial used for the positive electrode and a carbon-based activematerial used for the negative electrode, diffusion resistance is small.Therefore, even if the charging current changes, it is assumed that arelationship between the charging amount and an electromotive voltagedoes not largely change.

Meanwhile, in a case where a material with large diffusion resistancesuch as lithium titanate is used as an active material for the negativeelectrode, as shown in FIG. 5, a relationship between a charge amountand an electromotive voltage largely changes according to a currentvalue. Therefore, approximation similar to that of the positiveelectrode is not performed.

Accordingly, the negative electrode potential E_(a) is determined fromnumerical formula (11).

E _(a) =f _(a)(q/Q _(ia) ,I/Q _(ia))  (11)

Furthermore, the voltage V_(R) due to internal resistance is determinedfrom numerical formulas (12) and (13) using a charging current I andinternal resistance R(q).

V _(R) =R(q)×I  (12)

q=∫Idt  (13)

That is, numerical formula (2) is determined from numerical formula (14)below.

V _(c) =f _(c)(q/Q _(ic))−f _(a)(q/Q _(ia) ,I/Q _(ia))+R(q)×I  (14)

As described above, there is a nonlinear correlation between a chargingvoltage, and electromotive voltage characteristic and an internalresistance of the active material of the storage battery 2. Using thecapacity of an active material and the internal resistance as variables,regression calculation is performed on a characteristic curve withrespect to the charge amount of the charging voltage to derive thecapacity of the active material, the internal resistance, the capacityat the time of starting charging each active material, and the like.

The estimation part 6 estimates a volume change rate of the storagebattery 2, based on the estimated charge amount of the storage battery 2or the estimated charge amount of an active material used in the storagebattery 2, the charge amount of the storage battery 2 or the chargeamount of the active material used in the storage battery 2 (stored in astorage described later), and data indicating a volume change rate.

The storage 7 previously stores data indicating a relationship betweenthe charge amount of the storage battery 2 and the volume change rate ora relationship between the charge amount of an active material used inthe storage battery 2 and the volume change rate. For example, the datamay be stored as table data linking the charge amount to the volumechange rate. Stored data is not limited to the volume change rate, butmay be data indicating a relationship between the thickness data of thestorage battery 2 and the charge amount, and a relationship between thevolume change amount of the storage battery 2 and the charge amount. Theestimation part 6 estimates the volume change rate of the storagebattery 2 using data stored in the storage 7. Examples of the storage 7include a tape such as a magnetic tape or a cassette tape, a diskincluding a magnetic disk such as a floppy (registered trademark)disk/hard disk and an optical disk such as CD-ROM/MO/MD/DVD/CD-R, a cardsuch as an IC card (including a memory card)/optical card, and asemiconductor memory such as a mask ROM/EPROM/EEPROM/flash ROM.

Next, an example of operation of the secondary battery system accordingto the second embodiment will be described. FIG. 7 is a flowchartshowing an example of operation of the secondary battery systemaccording to the second embodiment.

First, information of a volume change rate of the storage battery 2 perunit time is designated to the designation part 5 (step 701). Asdescribed above, in a case where the charge controller 4 previously hasinformation of the volume change rate of the storage battery 2, thisstep is omitted.

The charge controller 4 causes a predetermined current to flow throughthe storage battery 2 (step 702).

The second measurement part 40 of the charge controller 4 measures acurrent, a charging voltage, temperature data, and the like of thestorage battery 2, and outputs the data to the estimation part 6 (step703). At this time, the second measurement part 40 preferably measurestime change of a charging voltage or the like for each unit battery.Measured data such as a charging voltage of each unit battery is storedin the RAM 62 or the storage 7. For example, N measured values areobtained during charging time t_(c) from start of charging untilreaching a charging end voltage.

The CPU 61 of the estimation part 6 executes the estimation program 641from the ROM 64 and analyzes a solution of a nonlinear differentialequation by regression calculation (step 704).

Since constant current charge is performed, q_(c) which is a chargeamount from the start of charging is determined by q_(c)=I·t bynumerical formula (13). In this way, measured values (V₁, q_(c1)), (V₂,q_(c2)), . . . , (V_(N), q_(cN)) corresponding to the charge amountq_(c) are obtained. The CPU 61 temporarily stores the obtained valueinto the RAM 62 or stores the obtained value into the storage 7.

Regression calculation is performed using the above measured values. Aresidual sum of squares used in performing the regression calculation isrepresented by the following numerical formula (15).

$\begin{matrix}{S = {\overset{N}{\sum\limits_{n = {{n\; s} + 1}}}\left( {V_{n} - \left( {{f_{c}\left( {\left( {q_{cn} + q_{0}^{c}} \right)/{Qc}} \right)} - {f_{a}\left( {{\left( {q_{cn} + q_{0}^{a}} \right)/{Qa}},{I/{Qa}}} \right)}} \right)} \right)^{2}}} & (15)\end{matrix}$

q₀ ^(c): charge amount of a positive electrode at the time of startingchargingq₀ ^(a): charge amount of a negative electrode at the time of startingcharging

The charge amount at the time of starting charging is unknown at thetime of the regression calculation. Therefore, the charge amounts of thepositive electrode and the negative electrode at the time of startingcharging are also unknown. In the second embodiment, in a case where thepositive electrode is a composite positive electrode of the activematerial A and the active material B, the unknown number of theregression calculation is represented by the following formula (16).

Q _(cA)

Q _(cB)

Q _(a)

q ₀ ^(cA)

q ₀ ^(cB)

q ₀ ^(a) R  (16)

As an initial value, an appropriate value, for example, a value at thelast measurement is used (step 705).

The simultaneous equations of the following numerical formula (17) aregenerated (step 706).

$\begin{matrix}\left\{ \begin{matrix}{{{\partial S}/{\partial Q_{cA}}} = 0} \\{{{\partial S}/{\partial Q_{cB}}} = 0} \\{{{\partial S}/{\partial Q_{a}}} = 0} \\{{{\partial S}/{\partial q_{0}^{cA}}} = 0} \\{{{\partial S}/{\partial q_{0}^{cB}}} = 0} \\{{{\partial S}/{\partial q_{0}^{a}}} = 0} \\{{{\partial S}/{\partial R}} = 0}\end{matrix} \right. & (17)\end{matrix}$

Each value of the next step is determined by the following numericalformula (18) (step 707).

$\begin{matrix}\left\{ \begin{matrix}\left. Q_{cA}\leftarrow{Q_{cA} + {\delta \; Q_{cA}}} \right. \\\left. Q_{cB}\leftarrow{Q_{cB} + {\delta \; Q_{cB}}} \right. \\\left. Q_{a}\leftarrow{Q_{a} + {\delta \; Q_{a}}} \right. \\\left. q_{0}^{cA}\leftarrow{q_{0}^{cA} + {\delta \; q_{0}^{cA}}} \right. \\\left. q_{0}^{cB}\leftarrow{q_{0}^{cB} + {\delta \; q_{0}^{cB}}} \right. \\\left. q_{cA}\leftarrow{q_{cA} + {\delta \; q_{0}^{a}}} \right. \\\left. R\leftarrow{R + {\delta \; R}} \right.\end{matrix} \right. & (18)\end{matrix}$

At this time, numerical formula (19) is represented as follows.

δQ _(cA)

δQ _(cB)

γQ _(a)

δq ₀ ^(cA)

δq ₀ ^(cB) δR  (19)

The numerical formula (19) can be obtained by solving the followingnumerical formula (20). Description is made using a Newton method in thesecond embodiment. However, another numerical analysis method such as aLevenberg method or a Marquardt method may be used instead thereof.

$\begin{matrix}{{\begin{pmatrix}\frac{\partial^{2}S}{\partial Q_{cA}^{2}} & \frac{\partial^{2}S}{{\partial Q_{cA}}{\partial Q_{cB}}} & \frac{\partial^{2}S}{{\partial Q_{cA}}{\partial Q_{a}}} & \frac{\partial^{2}S}{{\partial Q_{cA}}{\partial q_{0}^{cA}}} & \frac{\partial^{2}S}{{\partial Q_{cA}}{\partial q_{0}^{cB}}} & \frac{\partial^{2}S}{{\partial Q_{cA}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{{\partial Q_{cA}}{\partial R}} \\\frac{\partial^{2}S}{{\partial Q_{cA}}{\partial Q_{cB}}} & \frac{\partial^{2}S}{\partial Q_{cB}^{2}} & \frac{\partial^{2}S}{{\partial Q_{cA}}{\partial Q_{a}}} & \frac{\partial^{2}S}{{\partial Q_{cB}}{\partial q_{0}^{cA}}} & \frac{\partial^{2}S}{{\partial Q_{cB}}{\partial q_{0}^{cB}}} & \frac{\partial^{2}S}{{\partial Q_{cB}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{{\partial Q_{cB}}{\partial R}} \\\frac{\partial^{2}S}{{\partial Q_{cA}}{\partial Q_{a}}} & \frac{\partial^{2}S}{{\partial Q_{cB}}{\partial Q_{a}}} & \frac{\partial^{2}S}{\partial Q_{a}^{2}} & \frac{\partial^{2}S}{{\partial Q_{a}}{\partial q_{0}^{cA}}} & \frac{\partial^{2}S}{{\partial Q_{a}}{\partial q_{0}^{cB}}} & \frac{\partial^{2}S}{{\partial Q_{a}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{{\partial Q_{a}}{\partial R}} \\\frac{\partial^{2}S}{{\partial Q_{cA}}{\partial q_{0}^{cA}}} & \frac{\partial^{2}S}{{\partial Q_{cB}}{\partial q_{0}^{cA}}} & \frac{\partial^{2}S}{{\partial Q_{a}}{\partial q_{0}^{cA}}} & \frac{\partial^{2}S}{\partial q_{0}^{{cA}^{2}}} & \frac{\partial^{2}S}{{\partial q_{0}^{cA}}{\partial q_{0}^{cB}}} & \frac{\partial^{2}S}{{\partial q_{0}^{cA}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{{\partial q_{0}^{cA}}{\partial R}} \\\frac{\partial^{2}S}{{\partial Q_{cA}}{\partial q_{0}^{cB}}} & \frac{\partial^{2}S}{{\partial Q_{cB}}{\partial q_{0}^{cB}}} & \frac{\partial^{2}S}{{\partial Q_{a}}{\partial q_{0}^{cB}}} & \frac{\partial^{2}S}{{\partial q_{0}^{cA}}{\partial q_{0}^{cB}}} & \frac{\partial^{2}S}{\partial q_{0}^{{cB}^{2}}} & \frac{\partial^{2}S}{{\partial q_{0}^{cB}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{{\partial q_{0}^{cB}}{\partial R}} \\\frac{\partial^{2}S}{{\partial Q_{cA}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{{\partial Q_{cB}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{{\partial Q_{a}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{{\partial q_{0}^{cA}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{{\partial q_{0}^{cB}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{\partial q_{0}^{a^{2}}} & \frac{\partial^{2}S}{{\partial q_{0}^{a}}{\partial R}} \\\frac{\partial^{2}S}{{\partial Q_{cA}}{\partial R}} & \frac{\partial^{2}S}{{\partial Q_{cB}}{\partial q_{0}^{a}}} & \frac{\partial^{2}S}{{\partial Q_{a}}{\partial R}} & \frac{\partial^{2}S}{{\partial q_{0}^{cA}}{\partial R}} & \frac{\partial^{2}S}{{\partial q_{0}^{cB}}{\partial R}} & \frac{\partial^{2}S}{{\partial q_{0}^{a}}{\partial R}} & \frac{\partial^{2}S}{\partial R^{2}}\end{pmatrix}\begin{pmatrix}{\partial Q_{cA}} \\{\partial Q_{cB}} \\{\partial Q_{a}} \\{\partial q_{0}^{cA}} \\{\partial q_{0}^{cB}} \\{\partial q_{0}^{a}} \\{\partial R}\end{pmatrix}} = \begin{pmatrix}{- \frac{\partial S}{\partial Q_{cA}}} \\{- \frac{\partial S}{\partial Q_{cB}}} \\{- \frac{\partial S}{\partial Q_{ca}}} \\{- \frac{\partial S}{\partial q_{0}^{cA}}} \\{- \frac{\partial S}{\partial q_{0}^{cB}}} \\{- \frac{\partial S}{\partial q_{0}^{a}}} \\{- \frac{\partial S}{\partial R}}\end{pmatrix}} & (20)\end{matrix}$

It is judged whether a determined value satisfies a convergencecondition (convergence radius) of the following numerical formula (21)(step 708).

$\begin{matrix}\left\{ \begin{matrix}{{\delta \; Q_{cA}} < ɛ_{QcA}} \\{{\delta \; Q_{cB}} < ɛ_{QcB}} \\{{\delta \; Q_{a}} < ɛ_{Qa}} \\{{\delta \; q_{0}^{cA}} < ɛ_{q_{0}^{cA}}} \\{{\delta \; q_{0}^{cB}} < ɛ_{q_{0}^{cB}}} \\{{\delta \; q_{0}^{a}} < ɛ_{q_{0}^{a}}} \\{{\delta \; R} < ɛ_{R}}\end{matrix} \right. & (21)\end{matrix}$

In a case where the convergence condition is not satisfied (No in step708), an initial value is reset (step 705).

If the convergence condition is satisfied (Yes in step 708), theestimation part outputs an estimation result of a volume change ratio ofthe storage battery to the charge controller 4, based on estimated datasuch as a charging voltage or a charge amount of the storage battery 2,and data of a charge amount and the volume change ratio stored in thestorage 7 (step 709). For example, the estimation part 6 executesprocesses from step 704 to step 708 based on a current, a chargingvoltage, temperature data and the like of the storage battery measuredduring last charging, and stores a derived capacity of an activematerial, an internal resistance, a capacity at the time of startingcharging each active material, and the like into the storage or thelike. In a case where step 709 is executed based on the stored capacityof the active material, the internal resistance, the capacity at thetime of starting charging each active material or the like, theprocesses from step 704 to step 708 are omitted.

The charge controller 4 compares a threshold value input to thedesignation part 5 with the estimated volume change rate of the storagebattery 2 (step 710).

If the volume change rate of the storage battery 2 is equal to or largerthan the threshold value (Yes in step 710), the charge controller 4controls a current flowing through the storage battery 2 so as to besmaller than a predetermined current such that the volume change rate issmaller than the threshold value (step 711).

If the volume change rate of the storage battery 2 is smaller than thethreshold value (No in step 710), the charge controller 4 controls thecurrent flowing through the storage battery 2 so as to be larger thanthe predetermined current such that the volume change rate approachesthe threshold value (step 712).

The charge controller 4 judges whether the storage battery 2 is fullycharged (100% charge) (step 713).

If the charge controller 4 judges that the storage battery 2 is notfully charged (No in step 713), the process returns to step 703.

If the charge controller 4 judges that the storage battery 2 is fullycharged (Yes in step 713), the charge controller 4 stops supply of thecurrent to the storage battery 2 (step 714). Thereafter, the process isterminated.

The charge controller 4 only needs to judge whether the storage battery2 is fully charged at any time using an estimated value of the chargeamount of the estimation part 6 or the like. Alternatively, the chargecontroller 4 only needs to determine whether the storage battery 2 isfully charged by monitoring the charge amount at predetermined timeintervals.

In the secondary battery system according to the second embodiment, theestimation part 6 is configured separately from the storage 7, but thestorage 7 may be included in the estimation part 6. Furthermore, theestimation part 6 and the storage 7 may be included in the chargecontroller 4.

Use of the secondary battery system according to the second embodimentmakes it possible to obtain the same effect as the secondary batterysystem according to the first embodiment even without including thevolume measurement part 3.

In addition, the secondary battery system is space-saving because ofincluding no volume measurement part 3, and can have a simpleconfiguration.

Third Embodiment

The third embodiment will be described with reference to FIG. 8. FIG. 8is a block diagram showing an example of a secondary battery systemaccording to the third embodiment.

The secondary battery system 1 according to the third embodimentincludes a storage 70 storing data indicating a relationship between acharge amount of a storage battery (unit battery) and a thickness of thestorage battery. Other configurations are similar to those of thesecondary battery system according to the first embodiment.

The storage 70 stores data indicating a relationship between the chargeamount and the thickness of a unit battery in a case where apredetermined current flows. The stored data is not limited to thethickness of a storage battery 2 but may be a volume change rate or thelike. In addition, the above data is stored for each type of unitbattery. Examples of the storage 70 include a tape such as a magnetictape or a cassette tape, a disk including a magnetic disk such as afloppy (registered trademark) disk/hard disk and an optical disk such asCD-ROM/MO/MD/DVD/CD-R, a card such as an IC card (including a memorycard)/optical card, and a semiconductor memory such as a maskROM/EPROM/EEPROM/flash ROM.

FIG. 9 is a graph showing a relationship between a charge amount and athickness of a unit battery in a case where graphite is used for anegative electrode of the unit battery. The horizontal axis indicatesthe charge amount of the unit battery, and a value thereof isrepresented by 0 to 100%. The vertical axis indicates the thickness ofthe unit battery.

As shown in FIG. 9, in a case where graphite is used for the negativeelectrode of the unit battery, a change in the thickness of the unitbattery is classified into three sections of a low charge amount section(also referred to as an initial charging section or a first section), amedium charge amount section (also referred to as a medium chargingsection or a second section), and a high charge amount section (alsoreferred to as an end charging section or a third section). In the firstand third sections, a change rate of the thickness of the unit batteryis large. In the second section, the change rate of the thickness of theunit battery is small. A small change rate in thickness means that thevolume change rate is small. A large change rate in thickness means thatthe volume change rate is large. The first to third sections areconsecutive sections.

The volume change of a graphite material is caused by a stepwise changein a crystal structure (stage) of graphite according to the occlusionamount of lithium ions. The stages have different volume change ratesfrom one another, and the volume change rate changes as the stageproceeds. Therefore, if the charge amount (current value) per unit timeis small in the first and third sections where the volume change rate islarge, and if the charge amount (current value) per unit time is largein the second section, deterioration of the unit battery can be reducedwithout prolonging charging time. Furthermore, it is possible to shortenthe charging time while reducing deterioration of the unit battery. Acurrent flowing in the first to third sections may be set at a pluralityof constant current values.

The temperature of the unit battery rises when the battery is charged,and progress of deterioration of the battery is advanced when thetemperature further rises. Therefore, an average current value flowingin the third section is preferably smaller than an average current valueflowing in the first section. The average current value at this time iscalculated with respect to the charge amount and does not include acurrent value in a CV section (constant voltage section). A ratiobetween an average current value I₁ in the first section and an averagecurrent value I₃ in the third section preferably satisfies1.0<I₁/I₃<1.6, and more preferably satisfies 1.25<I₁/I₃<1.45.

Next, an example of operation of the secondary battery system accordingto the third embodiment will be described. FIG. 10 is a flowchartshowing an example of operation of the secondary battery systemaccording to the third embodiment.

First, the charge controller 4 acquires data related to the chargeamount and the thickness of the storage battery 2 from the storage 70(step 1001).

The charge controller 4 controls a constant current flowing in the firstsection so as to be smaller than a predetermined current based on theacquired data (step 1002).

The charge controller 4 monitors a change in the thickness of thestorage battery 2 measured by the volume measurement part 3 (step 1003).

Next, the charge controller 4 derives the charge amount based on acurrent value flowing through the storage battery and charging time, andjudges whether the charge amount corresponding to the first section hasbeen reached (step 1004).

If the charge amount corresponding to the first section has been reached(Yes in step 1004), the constant current flowing in the second sectionis set so as to be larger than the predetermined current (step 1005).

If the charge amount corresponding to the first section has not beenreached (No in step 1004), the process returns to step 1003.

The charge controller 4 monitors a change in the thickness of thestorage battery 2 measured by the volume measurement part 3 (step 1006).

Next, the charge controller 4 derives the charge amount based on acurrent value flowing through the storage battery and charging time, andjudges whether the charge amount corresponding to the second section hasbeen reached (step 1007).

If the charge amount corresponding to the second section has beenreached (Yes in step 1007), the constant current flowing in the thirdsection is set so as to be smaller than the predetermined current (step1008). At this time, the constant current flowing in the third sectionis preferably smaller than the current value which has flowed in thefirst section.

If the charge amount corresponding to the second section has not beenreached (No in step 1007), the process returns to step 1006.

The charge controller 4 monitors a change in the thickness of thestorage battery 2 measured by the volume measurement part 3 (step 1009).

Next, the charge controller 4 derives the charge amount based on acurrent value flowing through the storage battery and charging time, andjudges whether the charge amount corresponding to the third section hasbeen reached (step 1010).

If the charge amount corresponding to the third section has been reached(Yes in step 1010), the process is terminated.

If the charge amount corresponding to the third section has not beenreached (No in step 1010), the process returns to step 1009.

The magnitude of the current flowing in the first to third sections canbe appropriately changed according to charging time. In addition, thecurrent flowing in the first to third sections is not limited to aconstant current, but can be appropriately changed according to ameasurement result of the storage battery by the volume measurement part3.

FIG. 11 is a graph showing a result of a cycle test performed bychanging a ratio of (I₁/I₃) in a case where graphite is used for thenegative electrode of the storage battery. The horizontal axis indicatesa ratio (I₁/I₃) of an average current in the first and third sections,and the vertical axis indicates the number of cycles when the capacityretention ratio of the storage battery reaches 80%. A case where (I₁/I₃)is 1 indicates conventional CC-CV charging. Using charging time of CC-CVcharging as a reference, a current under each condition was set suchthat the charging time was equal. As shown in FIG. 11, in a case where(I₁/I₃) was 1.4, the cycle number was the largest.

FIG. 12 is a graph showing a relationship between the number of cyclesand a capacity retention ratio in cases of 0.7C-CCCV charging and0.8C-0.9C-0.6CCV charging.

As shown in FIG. 12, the capacity retention ratio with respect to thenumber of cycles is higher in a case of 0.8C-0.9C-0.6CCV charging. Thatis, when charging is performed, by setting a charging state including atleast three sections and setting an average current value in eachcharging state to (third section<first section<second section),deterioration of the storage battery can be reduced without prolongingcharging time.

In the above description, the case of including the first measurementpart 3 has been described, but in the third embodiment, the firstmeasurement part 3 can be omitted. With omission of the firstmeasurement part 3, processes in step 1003, step 1006, and step 1009shown in FIG. 10 can also be omitted.

When a charge amount of a storage battery is derived by setting thecharge amount in the first section to 0 to q_(x), the charge amount inthe second section to q_(x) to q_(y), the charge amount in the thirdsection to q_(y) to q_(z), the charge amount of the storage battery toq_(cn), and the capacity of a negative electrode (graphite) to Q_(a),the first section is represented by numerical formula (22).

$\begin{matrix}{\frac{\left( {q_{cn} + q_{0}^{a}} \right)}{Q_{a}} = q_{x}} & (22)\end{matrix}$

The charge amount of the storage battery in the first section is 0 to(Q_(a)×q_(x)−q₀ ^(a)) from numerical formula (22). Similarly, the chargeamount of the storage battery in the second section is (Q_(a)×q_(x)−q₀^(a)) to (Q_(a)×q_(y)−q₀ ^(a)) The charge amount of the storage batteryin the third section is (Q_(a)×q_(y)−q₀ ^(a)) to (Q_(a)×q_(z)−q₀ ^(a)).The initial charge amount q₀ ^(a) and the capacity Q_(a) of the negativeelectrode may be values previously stored in the storage, but arepreferably updated appropriately according to a charge curve analysismethod using the estimation part of the second embodiment or a batteryinternal state estimation method such as a dVdQ method.

Fourth Embodiment

The fourth embodiment will be described with reference to FIG. 13. FIG.13 is a diagram showing an example of a vehicle according to the fourthembodiment.

The vehicle of the fourth embodiment includes the secondary batterysystem according to any one of the first to third embodiments. Examplesof the vehicle herein include an automobile including an idling stopmechanism of two wheels to four wheels, a hybrid electric vehicle of twowheels to four wheels, an electric vehicle of two wheels to four wheels,an assist bicycle, and a train.

As shown in FIG. 13, in a vehicle 10 of the fourth embodiment, thesecondary battery system 1 according to any one of the first to thirdembodiments is mounted in an engine room. By disposing the secondarybattery system 1 in an engine room of the vehicle in a high temperatureenvironment, a distance from a battery pack to an electric drive systemdevice such as a motor or an inverter is shortened, loss of output andinput is reduced, and fuel efficiency is improved.

It is possible to provide the vehicle 10 including the secondary batterysystem 1 capable of exhibiting excellent cycle characteristics andcharging performance because the vehicle 10 includes the secondarybattery system 1 according to any one of the first to third embodiments.

The secondary battery system according to any one of the first to thirdembodiments can be used for electric products, sensors, domestic powerstorage systems, and the like without being limited to vehicles.

The secondary battery system according to any one of the first to thirdembodiments is also referred to as a storage battery system, a secondarybattery device, or a storage battery device.

In the secondary battery system according to any one of the first tothird embodiments, the designation part 5, the estimation part 6, andthe storages 7 and 70 may be included in an external server or the likefar from the storage battery 2. In this case, the charge controller 4may include a communication part and may control a charging current(electric power) or the like of the storage battery 2 by communicatingwith the external server.

While certain embodiments have been described, these embodiments havebeen presented by way of examples only, and are not intended to limitthe scope of the inventions. Indeed, the novel embodiments describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1-5. (canceled)
 6. A secondary battery system comprising: a secondarybattery having a charging state including at least first to thirdsections in a case where a predetermined current flows through thesecondary battery; and a controller that controls a current flowingthrough the secondary battery so as to be smaller than the predeterminedcurrent in the first and third sections, and controls the currentflowing through the secondary battery so as to be larger than thepredetermined current in the second section.
 7. The secondary batterysystem according to claim 6, wherein the controller controls an averagecurrent flowing through the secondary battery in the first section so asto be larger than an average current flowing through the secondarybattery in the third section.
 8. The secondary battery system accordingto claim 7, wherein the controller performs control such that an averagecurrent flowing in the first and third sections satisfies 1.0<I₁/I₃<1.6if the average current in the first section is represented by I₁ and theaverage current in the third section is represented by I₃.
 9. Thesecondary battery system according to claim 6, wherein the chargingstate indicates a relationship between a volume of the secondary batteryand a charge amount of the secondary battery in a case where thepredetermined current flows through the secondary battery.
 10. Thesecondary battery system according to claim 6, wherein the secondarybattery includes graphite in a negative electrode active material.
 11. Acharging method in a secondary battery having a charging state includingat least first to third sections in a case where a predetermined currentflows through the secondary battery, the charging method comprising:controlling a current flowing through the secondary battery so as to besmaller than the predetermined current in the first and third sections;and controlling the current flowing through the secondary battery so asto be larger than the predetermined current in the second section. 12.The changing method according to claim 11, wherein the controlling acurrent in the first and third sections further comprises controlling anaverage current flowing through the secondary battery in the firstsection so as to be larger than an average current flowing through thesecondary battery in the third section.
 13. The changing methodaccording to claim 12, wherein the controlling an average currentfurther comprises controlling such that an average current flowing inthe first and third sections satisfies 1.0<I₁/I₃<1.6 if the averagecurrent in the first section is represented by I₁ and the averagecurrent in the third section is represented by I₃.
 14. The changingmethod according to claim 11, wherein the charging state indicates arelationship between a volume of the secondary battery and a chargeamount of the secondary battery in a case where the predeterminedcurrent flows through the secondary battery.
 15. The charging methodaccording to claim 11, wherein the secondary battery includes graphitein a negative electrode active material. 16-17. (canceled)
 18. A vehiclecomprising: the secondary battery system according to claim 6; and anengine room in which the secondary battery system is disposed.